Charged particle beam systems are used in a variety of applications, including the manufacture, repair, and inspection of micro-fabricated devices, such as integrated circuits, magnetic recording heads, and photolithography masks. Charged particle beam systems may include electron beams, ion beams systems, or laser beams and may include more than one type of beam. Micro-fabrication typically includes creating or altering structures having very small dimensions, such as, for example, dimensions of tens of microns or less. As device geometries continue to shrink and new materials are introduced, the structural complexity of today's semiconductors grows exponentially allowing for smaller and smaller structures to be fabricated.
With such small structures it is necessary to direct the processing beam with great precision and accuracy. However, during processing the impact point of a beam on a sample tends to drift over time. For example, an operator may position the beam at point A at the beginning of a processing operation but the beam drifts to point B after a short time. The difference between the positions of point A and point B is referred to as beam drift. Beam drift can be caused by mechanical or thermal instabilities that cause slight movement of the stage on which the sample is supported or of the elements that generate and focus the beam.
A common method of accurately positioning a beam is to create a reference mark on the sample such as, for example, by milling. Such a reference mark is referred to as a fiducial. The beam is then positioned relative to the fiducial. A beam is initially directed to image the fiducial which is typically located near an area of the sample to be processed, typically referred to as a region of interest (ROI). A vector between the ROI and the fiducial is determined and the fiducial is then used to track the beam position during the processing of the ROI. The fiducial is designed in a shape that is recognizable by an image recognition program allowing automatic positional tracking of the fiducial, and therefore the ROI. The beam periodically images the fiducial and any drift of the beam is corrected. During processing the beam is typically repeatedly scanned across the ROI and may be programmed to have variable dwell time of the beam on the ROI. Due to the high kinetic energy of particles in the beam any exposure of the sample surface surrounding the ROI to the beam tends to damage the surface through etching. Additional damage to the surface may occur during imaging as the beam is scanned across the surface. Eventually, the fiducial becomes so damaged that it is no longer recognizable by image recognition, and therefore is no longer useable for positional tracking.
FIGS. 1A-1B illustrate a commonly used method of correcting beam drift using fiducials. FIG. 1A shows a region of interest 104 on the surface of a sample. Fiducials 106 are created proximal to the region of interest 104 to allow for positional tracking of the region of interest 104. Image frame 102 depicts the boundary of the area scanned by the beam during imaging and includes fiducials 106 and region of interest 104. The interior of image frame 102 is the area scanned with the beam when imaging, and therefore receives damage from beam exposure. FIG. 1B shows image frame 102 after processing with the beam for a period of time. Due to exposure to the beam, fiducials 106 become damaged as seen at 108 and are no longer identifiable by image recognition. At this point it is no longer possible to track the location of the beam relative to region of interest 104.
A common use of charged particle processing is the creation of thin specimens for viewing in a transmission electron microscope (TEM). Several techniques are known for preparing TEM specimens. Techniques generally referred to as “lift-out” techniques use focused ion beams to cut the sample from a substrate or bulk sample without destroying or damaging surrounding parts of the substrate. Such techniques require precise positioning of the beam. To automate lift-out techniques, the position of the beam relative to the sample should be automatically ascertainable with a high degree of accuracy, and the degradation of a fiducial reduces the accuracy.
Such techniques are useful in analyzing the results of processes used in the fabrication of integrated circuits, as well as materials general to the physical or biological sciences. These techniques can be used to analyze samples in any orientation (e.g., either in cross-section or in plan view). Some techniques extract a sample sufficiently thin for use directly in a TEM; other techniques extract a “chunk” or large sample that requires additional thinning before observation. In addition, these “lift-out” specimens may also be directly analyzed by other analytical tools, other than TEM. Techniques where the sample is extracted from the substrate within the focused ion beam (FIB) system vacuum chamber are commonly referred to as “in-situ” techniques; sample removal outside the vacuum chamber (as when the entire wafer is transferred to another tool for sample removal) are call “ex situ” techniques.
What is needed is an improved method of creating a fiducial on a sample for positional tracking of a region of interest during processing by a charged particle beam system.